A number of types of motion sensitive circuits became known which can be grouped into two main groups according to the connection of the switching elements sensing the motion or dislocation. The first group includes the so-called rest current-type sensors. The sensors employed in such group include such mechanical switching elements which in their rest position are in a closed circuit position and upon dislocation or movement, will go over into an open circuit position, consequently, the change of state of the sensors is indicated by the interruption of the current which used to flow through them.
The second group includes the so-called operating current-type sensors which are connected parallel with respect to each other and are connected to a voltage source. The sensors employed in this group are open circuited switches in their rest position, while upon dislocation or movement, the voltage appearing across their terminals becomes shorted. The current which starts to flow through the loop will indicate that the sensors have undergone a change of state.
Both groups have a common characteristic in that all the sensing switches are similar and in their alarm state, that is, in their dislocated or moved condition, they will prevent the operation of the remaining switches.
The reliability of property protecting circuits requires that the sensors of a protecting system should be able to operate independently from each other because with such possibility, the protection is greatly enhanced. Intruders or unauthorized persons can easily disarm or put out of operation a system which is sensitive only to a single intrusion and, thereafter, such system is unable to offer any kind of protection whatsoever against a repeated unauthorized intrusion.
Switching constructions or sensors became known which operate independently from each other and according to such constructions the switching elements or sensors are connected individually to independent registering units or an appropriately coded signal transmission has been incorporated into them. Both of such structures make the switching operation as well as the location and the control of the devices complicated. For this reason such systems did not enjoy a wide acceptance.
It is a known and presently widely used solution according to which each of the switching elements placed in a sensing loop, operating with a rest current, are shunted by a resistance having a certain magnitude. As a result, the change of state of the individual switching elements is indicated not by the interruption of the current, but by the gradual decrease of the loop current. The disadvantage of such constructional solution resides in that upon the activation of a few sensing elements or in the event of their defect, the loop current will be reduced to such an extent that the registering unit will not be able to perform its required function.